1. Field of the Invention
The present invention relates to a thermal transfer film comprising a coloring layer formed on a substrate film via an intermediate layer, and more particularly relates to a thermal transfer film which can give a clear printing without lack of impression to a paper to be printed and allows reliable coating of an intermediate layer and a coloring layer on a substrate film and has secret leakage preventing properties, and an image forming method using the same.
2. Description of the Related Art
Conventionally, thermal transfer films comprising a coloring layer comprising a thermally fusible ink formed on one side of a substrate film have been used as thermal transfer recording media used for thermal transfer printers, facsimiles and the like.
Conventional thermal transfer films have substrate films made of about 10 to 20 xcexcm thick papers such as condenser paper and paraffin paper or about 3 to 20 xcexcm thick plastic films such as polyester and cellophane and coloring layers obtained by coating on the substrate film thermally fusible inks which are mixtures of binders, colorants such as pigments and dyes, and additives such as melting point-lowering agents and plasticizers as occasion demands. Some thermal transfer films have intermediate layers adjusted to melt by the energy for printing between the substrate films and coloring layers.
The substrate films are heated and pressed in the predetermined areas by thermal heads from behind to melt the coloring layers corresponding to the printing areas to transfer the same onto transfer receiving materials for printing.
However, when the conventional thermal transfer films having intermediate layers and thermally fusible coloring layers formed on substrate films are used for printing, there have been problems that letters and fine lines are blurred by lack of impression to give the printed materials a patchy appearance and that there is a large noise emitted when the thermal transfer films are separated from transfer receiving materials. In order to print without lack of impression on coarse papers with a Beck smoothness of 50 seconds or lower, it is necessary to transfer all the coloring layer without occurrence of lack of impression to the printed materials (without being left on the intermediate layer) in the areas to which energy is applied according to the pixel by a means such as a thermal head to a transfer receiving paper. It is effective to separate the transfer receiving material from the thermal transfer film when the intermediate layer of the thermal transfer film, which has a coloring layer via the intermediate layer on the substrate film, is melted and flowable, and therefore in a liquid state in order to transfer all the necessary coloring layers to the transfer receiving paper. However, there is a time interval between the time when the transfer receiving material and the thermal transfer film are superimposed and the printing energy is applied to the thermal transfer film and the time when the thermal transfer film is separated from the transfer receiving material in machines generally used such as facsimiles using thermal transfer films. There arises a disadvantage in that the intermediate layer is cooled and solidified or decreases in flowability if not solidified in the time interval even when the intermediate layer is adjusted to melt by the printing energy.
Incidentally, materials having so-called supercooling properties, which have freezing points 10xc2x0 C. or lower than their melting points, are known in various literatures. Techniques about thermal transfer films having coloring layers on the substrate films via intermediate layers comprising various materials having supercooling properties are known. For example, such techniques are disclosed in Japanese Patent Application Laid-Open Nos. 61-235189, 61-286195, 62-9991, 62-82084, 63-302090, 3-246094 and others. On the other hand, it is well known that polycaprolactone-based resins have supercooling properties in various literatures. Techniques about thermal transfer films having coloring layers containing the polycaprolactone-based resin formed on the substrate films are well known. For example, such techniques are disclosed in Japanese Patent Application Laid-Open Nos. 59-230795, 60-122194, 60-122195, 61-185492, 62-59089, 5-32073 and others.
Furthermore, techniques about thermal transfer films having coloring layers via intermediate layers containing the polycaprolactone-based resin formed on the substrate films are well known. For example, Japanese Patent Application Laid-Open No. 60-165291 discloses that the polycaprolactone-based resin is used in an intermediate layer for the purpose of multiple printing and Japanese Patent Application Laid-Open No. 7-232483 discloses that polycaprolactone with a molecular weight of 10,000 or less is used in a primer layer for the purpose of facilitating high-speed printing and smooth printing in a high temperature atmosphere.
However, the thermal transfer films employing the intermediate layers according to these techniques still have the disadvantage in that letters and fine lines are blurred by lack of impression to give the printed materials a patchy appearance. What is worse, the thermal transfer films have a disadvantage in that when the ink for forming the intermediate layer is coated on the substrate film, the intermediate layer material stays melted by the heat for drying for a while even after the intermediate layer ink has been heated and dried, which undesirably causes adhesion between the substrate film side of the thermal transfer film wound after the coating and the intermediate layer side. Moreover, when a coloring layer is coated on the substrate film having an intermediate layer formed thereon, the hot-melt coating method, which facilitates the low-cost coating because no solvent is needed, has a disadvantage in that polycaprolactone present in the intermediate layer is melted and becomes fluid by the heat of the heated and melted coloring layer ink, which prohibits the coloring layer ink from being coated with a good surface quality.
It is an object of the present invention to solve the above-mentioned drawbacks and problems to provide a thermal transfer film which can give a clear printing without occurrence of lack of impression in the printed paper and allows reliable coating of an intermediate layer and a coloring layer on a substrate film and which has secret leakage preventing properties and an image forming method using the same.
In order to achieve the object, the inventor has investigated melt viscosities of thermally fusible substances having supercooling properties which are in a melted state when the transfer receiving material and the thermal transfer film are separated and has completed the present invention about the thermal transfer film. Furthermore, the inventor has evaluated coating suitability of intermediate layers containing thermally fusible substances having supercooling properties and overcoating suitability of coloring layers onto intermediate layers, and has identified a series of binder resins which can improve the both suitabilities without adversely affecting melt viscosities of thermally fusible substances having supercooling properties, and has completed the present invention about the thermal transfer film. Furthermore, the inventor has measured and examined time intervals between the time when the thermal transfer film and the transfer receiving material are superimposed and heated to record and the time when the two materials are separated and has completed the present invention about the image forming method in which lack of impression does not occur in the printed material and a clear printing is possible.
Accordingly, the thermal transfer film according to the present invention is a thermal transfer film having a coloring layer via an intermediate layer formed on a substrate film, wherein the intermediate layer contains a thermally fusible substance and a non-transferable binder resin, and the melt viscosity of the thermally fusible substance in the temperature range 15 to 25 xc2x0 C. higher than the fuse peak temperature (the fuse peak temperature defined in JIS K 7121-1987) of the thermally fusible substance is 100 mPaxc2x7s or more and 1000 mPaxc2x7s or less, and the fuse peak temperature (the fuse peak temperature defined in JIS K 7121-1987) of the thermally fusible substance is in the range of 50 to 110xc2x0 C., and the crystallization peak temperature (the crystallization peak temperature defined in JIS K 7121-1987) of the thermally fusible substance is in the range of xe2x88x9220 to 100xc2x0 C., and the crystallization peak temperature of the thermally fusible substance is lower than the fuse peak temperature by 10xc2x0 C. or more, and the softening temperature of the binder resin (the softening temperature measured by the ring and ball method defined in the JIS K 2207-1980) is 130xc2x0 C. or more and 400xc2x0 C. or less.
Furthermore, it is preferable that the binder resin is incompatible with the thermally fusible substance.
Furthermore, it is preferable that the intermediate glass transition temperature (the intermediate glass transition temperature defined in the JIS K 7121-1987) of the binder resin is higher than the fuse peak temperature (the fuse peak temperature defined in the JIS K 7121-1987) of the thermally fusible substance by 2xc2x0 C. or more.
Furthermore, it is preferable that the binder resin has a number average molecular weight of 8,000 or more and 1,000,000 or less, and has a benzene ring structure, and is a polyester resin.
Furthermore, it is preferable that the intermediate layer has a carbon black incorporated therein.
Furthermore, it is preferable that the binder resin forms a porous membrane which is not thermally transferable, and that the porous membrane has the thermally fusible substance contained in the pores, and that a carbon black is incorporated in the porous membrane.
Furthermore, it is preferable that the melt viscosity of the coloring layer at 100xc2x0 C. is 150 mPaxc2x7s or more and 300 mPaxc2x7s or less, and that the difference between the fuse peak temperature (the fuse peak temperature defined in the JIS K 7121-1987) of the coloring layer and the fuse peak temperature (the fuse peak temperature defined in the JIS K 7121-1987) of the thermally fusible substance is 10xc2x0 C. or less.
The image forming method of the present invention comprises steps of:
providing the above-mentioned thermal transfer film according to the present invention;
providing a transfer receiving material;
superimposing the transfer receiving material on a coloring layer side of the thermal transfer film;
heating and recording from the substrate film side according to the pixel by heating means and separating the thermal transfer film and the transfer receiving material,
wherein a time interval between recording each pixel and separating the thermal transfer film and the transfer receiving material is 2 seconds or less.
Furthermore, it is preferable that the heating means is a thermal head of an entire surface glaze or a partial glaze.
In the above-described present invention, even if the intermediate layer made of a thermally fusible substance and a specific binder resin is cooled to some degree in the areas where the printing energy has been applied in the time interval between printing and separation, the interface with the coloring layer remains melted and is low in viscosity due to the supercooling properties of the components, which allows the coloring layer to transfer from the thermal transfer film to the transfer receiving material with a low stripping force and prevents the coloring layers in the areas where the printing energy has been applied from undergoing cohesive failure in the layer and from being left on the intermediate layer. This enables all the coloring layer in the areas where the energy has been applied to transfer to the transfer receiving material and thus a good printing with little patchiness can be obtained even when a rough paper is used.
JIS K 7121-1987 teaches testing methods for determining transition temperatures (melting temperatures, crystallization temperatures, and glass transition temperatures) of plastics. Each method teaches measuring the difference in temperature between a test specimen and a reference substance as a function of temperature while varying the temperatures of the test specimen and reference substance according to a controlled programme.
Where the melting temperature is to be determined, preliminary maintain the reference substance until it stabilizes at a temperature about 100xc2x0 C. lower than the melting temperature of the test specimen, and then heat the reference substance to a temperature about 30xc2x0 C. higher than that at the end of melting peak at a heating rate of 10xc2x0 C. per minute, while measuring the temperature difference between the reference substance and the test specimen, to obtain a melting curve. The melting peak temperature (fuse peak temperature) is defined as the crest of the melting peak on the melting curve.
Where the crystallization temperature is to be determined, heat the reference substance to a temperature about 30xc2x0 C. higher than that at the end of melting peak, and after maintaining this temperature for 10 minutes, cool to a temperature about 50xc2x0 C. lower than that at the end of crystallization peak at a cooling rate of 5xc2x0 C. or 10xc2x0 C. per minute, while measuring the temperature difference between the reference substance and the test specimen to obtain a crystallization curve. The crystallization peak temperature is defined as the crest of the crystallization peak on the crystallization curve.
Where the glass transition temperature is to be determined, preliminarily maintain the reference substance until it stabilizes at a temperature about 50xc2x0 C. lower than the transition temperature, and then heat the reference substance to a temperature about 30xc2x0 C. higher than that at the end of transition at a heating rate of 20xc2x0 C. per minute, while measuring the temperature difference between the reference substance and the test specimen, to obtain a glass transition curve.
Find the mid-point temperature of glass transition as the temperature at the intersection of the straight line equidistant in the vertical axial direction from the straight lines formed by extending the respective base lines and the curve showing a stepped change of glass transition.
Find the extrapolated onset temperature of glass transition (intermediate glass transition temperature) as the temperature at the intersection of the straight line formed by extending the base line on the low temperature side to the high temperature side and the tangent line drawn to the curve showing a stepped change of glass transition at a point of maximum gradient.
JIS K 2207-1980 teaches a test method for determining the softening point of a test specimen. The method is carried out by casting a sample in specified rings, supporting them horizontally in a water or glycerin bath and place a specified weight ball on the centre of each ring. The softening temperature is defined as the temperatures at which the sample softens, sags downwards and touches the bottom plate of the ring holder by the weight of a steel ball, when the bath temperature is raised at a specified rate. The ring shall be a shouldered ring made of brass or brass with nickel or chrome plating. The ball shall be a xe2x85x9c (dia. 9.525 mm) Ordinary Class ball with the mass of 3.5+0.05 g. The guide shall be made of brass or brass with nickel or chrome plating. The ring holder shall be made of brass or brass with nickel or chrome plating and capable of supporting a thermometer and rings as described below.
Rings can be supported horizontally under such a position that the distance between the upper surface of the ring and the upper edge of the heating bath is 75 mm and over, and the distance between the former and the bath liquid surface is 50 mm and over downwards, respectively.
The distance between the lower surface of the ring and the upper surface of the bottom plate of ring holder shall be 25.4 mm, and the bottom plate shall be 12.7 to 19.1 mm above the bottom of the heating bath.
The thermometer shall be supported at the position whose bottom end of the bulb is the same horizontal plane as the lower surface of the ring, and within 10 mm from the ring, without touching the sample shelf.